Field of Invention
The present invention relates to an improved process to remove hydrogen sulfide and hydrogen polysulfide from liquid sulfur (sulphur). Hydrogen sulfide is removed by mass transfer to air bubbles generated by passing air through a perforated plate. A volatile catalyst is used to rapidly decompose hydrogen polysulfide to hydrogen sulfide.
Description of the Related Art
The Claus process is extensively used to produce liquid sulfur from sour oil and gas or other gaseous hydrogen sulfide. It is known in the industry that produced sulfur contains two hydrogen sulfide species, namely, physically dissolved hydrogen sulfide, H2S, and hydrogen polysulfide, H2Sx, which is the reaction product of dissolved H2S with liquid sulfur. Liquid sulfur produced in processing plants using the Claus process may contain upwards of 500 parts per million by weight (ppmw, where H2Sx is reported as H2S equivalent).
The prior H2S-sulfur system involves two coupled reversible reactions, namely the physical dissolution of H2S in liquid sulfur, which decreases with temperature, (represented by reversible reactions (1 below), where H2S(g) denotes H2S in the gas phase and H2S(d) denotes H2S dissolved in liquid sulfur) and the existence of a further reversible reaction between dissolved H2S and liquid sulfur (2 below), which increases with temperature.
                              H          2                ⁢                  S                      (            g            )                          ⁢                                                            ⟶                                  k                  1                                                                                        ⟵                                      ⁢                  H          2                ⁢                  S                      (            d            )                                              (        1        )                                                      H            2                    ⁢                      S                          (              d              )                                      +                              S                          x              -              1                                ⁢                                                                      ⟶                                      k                    2                                                                                                                        ⟵                                      k                                          -                      2                                                                                                    ⁢                      H            2                    ⁢                      S            x                                              (        2        )            
After production in Claus plants, dissolved H2S spontaneously degasses into the headspace of tanks and/or vessels found throughout the handling, storage and transportation chain (e.g., pits, tanks, railcars, truck tankers, and the like). Over time, the concentration in the gas phase may reach toxic or explosive levels. The lower explosive limit in air is approximately 4% by volume; the lethal concentration is approximately 600 parts per million by volume (ppmv); In addition, nuisance odor may result from fugitive emissions in concentrations lower than 0.001 ppmv. A need to degas sulfur is apparent in view of the potential accumulation hazardous levels of H2S in the handling chain. The degassing benchmark generally adopted by the industry (typically to avoid reaching explosive levels during transportation) is 10 parts per million by weight.
The principal considerations with respect to industrial degassers involve the rates at which dissolved H2S is transferred from the liquid phase into the gas phase (reaction −1) and the decomposition of H2Sx (reaction −2). The degassing rate of dissolved H2S may be maximized by (a) producing a large surface for gas-liquid mass transfer and (b) reducing the boundary layer next to the interface through which the dissolved H2S diffuses to reach gas-liquid surface. Large surface area may be created by generating a myriad of fine sulfur droplets, generating a myriad of bubbles of stripping gas, or packing. In methods using bubbles, the preferred stripping gas in most instances is air because it is cheaper than inert gases or steam, plus air has the additional benefit that a portion of the H2S and H2Sx is consumed by reacting with oxygen (reaction products are sulfur and water). Agitation or circulation is almost always part of the process since this enhances the rate of diffusion of dissolved H2S through the liquid boundary layer surrounding the bubbles.
Whereas dissolved H2S evolves directly to a gas phase, H2Sx generally does not. The process whereby H2Sx is removed is typically via a first decomposition to dissolved H2S (reaction −2), followed by mass transfer of dissolved H2S degasses across the gas-liquid boundary (reaction −1). The decomposition reaction tends to be very slow, such that H2Sx persists as a source of H2S gas for a long time.
The slow decomposition of H2Sx represents a main obstacle in the degassing processes. For this reason, various degassing processes make use of a catalyst to accelerate the decomposition reaction. Catalysts may be liquid or solid (generally, a bed of granules). Many different chemical types have been used, including amines. In the past, use of amines fell into disfavor by the industry because solid sulfur, subsequently solidified, was unacceptably friable which resulted in a very dusty product.
Once the H2S is transferred to the gas phase, it may be removed from the degasser using various removers, such as fans, eductors, and the like. The effluent containing the H2S extracted from the liquid sulfur may be delivered to an incinerator, a tail gas treatment unit or back to front end of the Claus processing plant.
Numerous degassing patents have been granted, some of which are relevant based on the method used with regards to gas-liquid contact (sparging) and the use of amine-type liquid catalysts. Whereas the degassing rate is critically dependent on the efficiency of sparging, patents that disclose a sparging gas provide little to no description of the sparging apparatus. Illustrative examples from relevant patents are identified below.
U.S. Pat. No. 4,729,887 (Pendergraft) discloses a vessel which is a concrete pit with 3 cells. The middle cell contains a bed of alumina or cobalt-molybdenum impregnated alumina (solid catalyst). Air is delivered to distributor manifold provided with a plurality of perforated pipes under the catalyst bed. Air assists in circulating sulfur through the bed.
U.S. Pat. No. 5,935,548 (Franklin) discloses a system where sulfur is agitated and mixed using an eductor supplied with partly degassed sulfur (which agitates/mixes liquid sulfur). Air is supplied through a pipe and discharged (a) in the vicinity of the eductor (b) into the stream of partly degassed sulfur or (c) to a “sparger” underneath the eductor(s). The sparger appears to consist of a pipe provided with openings. The diameter of the pipe or openings therein is not specified.
U.S. Pat. No. 6,149,887 (Legas) discloses an apparatus consisting of various arrangements of cells and baffles. Heated gas is fed to distributors in each cell. Franklin purports to generate finely divided gas bubbles using tubes with a multiplicity of small openings.
U.S. Pat. No. 6,676,918 (Wu) discloses a method to degas in Claus rundown seal pots. Compressed air is injected under pressure via a line having a small opening nozzle into the annular space of the seal pot.
US patent application 2011/0182802 A1 (Garg) discloses a system that supplies compressed air to a gas diffuser located below a packing. The difusser has a predetermine shape and size and is provided with holes ¼ of openings in the packing. Use of a sintered metal diffuser may also be used.
Canadian Patent No. 2,170,021 (Ellenor) discloses up to four cells equipped with an impeller/shroud assembly. Air is ingested into liquid sulfur by the impeller and the mixture then passed through a perforated shroud creating small bubbles to aerate the cell. High turbulence combined with tiny bubbles results in very fast degassing. A mixture of morpholine and cyclohexylamime is added to catalytically decompose H2Sx. The last cell is dedicated to the removal, by degassing, of the volatile catalyst. Doing so eliminates the objection of producing friable (solid) product.